This invention relates to image display panels. It is particularly directed to a highly efficient cathodoluminescent panel useful for image displays such as alphanumeric and computer graphics, and well suited to television displays.
Although the field of the gas discharge display panel has been diligently explored, no device has yet been able to meet the standards of performance and cost as high as those established by the cathode ray television picture tube in its current state of development.
Ideally, the gas discharge display panel offers many benefits. First of all, it is not size-limited as stringently as the picture tube, wherein any increase in picture area much greater than the twenty-five inch diagonal measure results in an inordinate increase in bulk and weight. For example, a picture tube with a twenty-five inch diagonal measure weighs about fifty pounds while a tube with a thirty inch diagonal measure may weigh more than a hundred pounds. To cite other advantages, flat panel displays, which are commonly built in a matrix of linear rows of columns of discrete picture elements, are inherently capable of producing pictures of near-perfect raster linearity, interlace and color field registration. But these theoretical benefits have been largely offset by undesirable performance characteristics such as inadequate brightness, low luminous efficiency, luminance non-uniformity, and lack of contrast.
Of these problems, inadequate brightness and low luminous efficiency have proved to be among the most troublesome impediments to commercial viability. The maximum level of brightness produced by a discrete picture element in prior art gas discharge panels has been but a fraction of the brightness level of an equivalent picture element in a television picture tube. As a consequence, it has not been feasible to scan gas discharge picture elements point-at-a-time as is done in the picture tube and yet achieve an acceptable brightness level. However, a greater level of brightness can be obtained in gas discharge panels by operating a full line of display panel picture elements at a time. Even by this expedient, however, a brightness level adequate for comfortable television viewing, and competitive with the current television picture tubes, has not been shown.
Luminance non-uniformities have also proved troublesome in display panels. This problem may manifest itself as spots, rings or striations of light brighter than surrounding areas of the image field. These manifestations may remain fixed, or move about. Since the human eye is particularly sensitive to even slight differences in luminance intensity, the effects can prove deleterious, especially in panels used for the reproduction of images having a full gray scale, such as the television picture.
Operation of the gas-discharge display panel is based upon the principles of the widely known glow-discharge tube, an example of which is shown by FIG. 1. Enclosed within an evacuated envelope 12 is cathode 14 and an anode 16. Envelope 12 may contain one of the noble gases such as krypton or argon, or common gases such as nitrogen, hydrogen, mercury vapor, or a mixture thereof. A suitable potential applied between cathode 14 and anode 16 results in a glow discharge within the envelope. The entity exhibits classic gas discharge phenomena including a cathode dark space 20, a negative glow 22, a Faraday dark space 24, and a positive column 26.
FIG. 2 shows an element of a prior art gas discharge display panel for producing spots of light utilizing the medium of the gas discharge tube. In essence, an intermediate apertured insulator 30 is located in a positive column 32 of a gas discharge cell 33. A "plasma sac" 34 (also called an "electrostatic double layer" in the art) forms on the cathode side of the aperture 31. Primary electrons from the cathode 35 generate secondary electrons in the gas discharge and are gathered by the plasma sac 34 and channeled into aperture 31. Light visible to the viewer, indicated by 36, is produced within sac 34 due to the higher electron temperature within the sac as compared to the electron temperature outside the sac. The phenomenon is described in a journal article entitled "A Picture-Display Panel Using a Constricted Glow Discharge", by H. Hori et al, IEEE transactions on Electron Devices, Vol. ED-21, No. 6. June, 1974. A gas discharge display apparatus utilizing the plasma sac is disclosed by Miyashiro et al in U.S. Pat. No. 3,749,969.
Further with regard to FIG. 2 and the concept it represents, it is said that a gas such as neon can be used at a nominal pressure of five torr. An intermediate electrode 38 plated inside aperture 31 is used for propagation of the plasma sac 34 to an adjacent aperture having a similar intermediate electrode (not shown). Propagation is due to a priming effect in that the presence of the discharge in one aperture lowers the breakdown voltage of a discharge in an adjacent aperture to encourage the formation of a plasma sac in that aperture. At the same time, the discharge in the first cell is switched off. Thus, by this scheme, point-by-point scanning can be obtained, and as luminance is a linear function of current, the intensity of the light 36 can be varied so intermediate values of gray of limited scope can be obtained.
Another approach followed by the prior art is to utilize ultraviolet emissions emanating from a positive column to stimulate the emission of light in the visible spectrum.
A phosphor is disposed on the transparent walls of the cavity surrounding the positive column. One execution of this approach utilizes the plasma sac phenomenon as described in a journal article by H. Hori et al (Op. cit.), but alleges to be an improvement thereon in that it is said to utilize a more efficient ultraviolet excitation of phosphors from a positive column, rather than a negative glow luminance light production phenomenon taught by Hori. The method of ultraviolet excitation of phosphors is described in "Electron Accelerating Display Cell," Y. Okamoto et al, Preprint Number 464 of the 1975 national meeting of the Institute of Electrical Engineers of Japan, 1975.
According to the referenced document, the device is said to be operable in three modes, as illustrated by FIGS. 2A, 2B, and 2C. It will be observed that the gas discharge cell structures in the figures are identical, in that each has a cathode 1 and associated negative glow, an anode 2 and intermediate electrode 3 having an aperture therein, and a phosphor 4 disposed on an inner surface of evacuated, gas-charged envelope 5. The particular mode that develops in these common configurations depends upon the potential on anode 2; that is, a progressive increase in levels of potential on anode 2 results in modes I, II, and III as shown in FIGS. 2A, 2B, and 2C respectively. Mode II is the favored mode and would seem to be the most feasible mode of operation, with modes I and II considered as not being viable for display applications.
With regard to the operation of mode I illustrated by FIG. 2A, electrons generated in the interspace between cathode 1 and intermediate electrode 3 diffuse into display cell area 6 and are accelerated by relatively low potential on anode 2, said to be on the order of 200 volts or less. An electron e may follow a typically random collison-determined path 7 to impinge upon low-voltage phosphor 4, causing emission of light 8. Alternatively, an electron e following path 9 may collide with an atom 11, resulting in the emission of ultraviolet light which, upon impact with phosphor 4, also results in the emission of visible light 8A. It appears that no positive column is generated in mode 1 operation.
With regard to FIG. 2B and mode II operation, a relatively greater potential on anode 2, assumed to be more than 200 volts, results in the formation of a positive column 13 which emits abundant ultraviolet light for the excitation of phosphor 4. When a predetermined threshold level is reached in the discharge current, a plasma sac 15 forms. Plasma sac 15 provides for amplification of the electron current drawn through the aperture in intermediate electrode 3, providing for greater phosphor excitation than mode I through enhanced ultraviolet emission from positive column 13.
In mode III operation shown by FIG. 2C, a potential said to be even greater on anode 2 causes the intermediate electrode 3 to become a second cathode, as shown by the presence of a second negative glow 17. A positive column 19 also appears, resulting in photoluminescence as in mode II. The presence of the negative glow 17, however, causes the discharge current to "run away," as a self-sustained discharge develops between intermediate electrode 3 and anode 2 as in an ordinary gas discharge. To prevent this run-away condition a resistive entity (not shown) must be placed in series with the intermediate electrode 3. This in turn reduces the panel speed response as compared with preferred mode II. (See "A New DC Gas Discharge Display With Internal Memory," Y. Okamoto et al. Japan J. Appl. Phys. Vol. 15 (1976), No. 4; also, Patent Disclosure No. 26 01 925 (German).
Schwartz, in U.S. Pat. No. 3,845,241 discloses a gas discharge structure as a source of free electrons which are accelerated in an adjoining, second section into impingement with an electron-excitable phosphor screen. Establishment of a gas discharge in the second section is precluded by appropriate selection of certain dimensions, gas pressure and accelerating voltage according to Paschen's law. Moderating means are provided in certain embodiments for causing the energy range of free electrons entering the second section to be narrow relative to the range of energies of free electrons generated in the gas discharge. A number of control grid arrangements are also disclosed.
Displays in which a light-emissive material is directly excited by electron bombardment are known as cathodoluminescent displays. Obtaining an adequate number of electrons for adequate excitation of the light-emissive material, and hence adequate brightness, has been a problem in panel displays utilizing cathodoluminescence, as the standard planar cathode in its present state of development does not yield enough electrons at low gas pressures for an effective display. To remedy this deficiency, a structure known as a "hollow cathode" has been introduced into cathodoluminescent panel displays. The use of a hollow cathode in gas discharge displays and the advantages thereof, are disclosed in U.S. Pat. Nos. 3,992,644 3,938,135 and 3,999,094 assigned to the assignee of the present invention.